1. Field of the Art
This invention relates to a heating furnace of the natural convection type in which an article is heated while it passes through the furnace.
2 Description of the Prior Art
A heating furnace of the type may have a heat source which is suitable from the viewpoints of heating efficiency, stability, uniformity and safety. The heating furnace which is heated by burning a liquid or gaseous fuel by a burner and introducing the combustion gas directly into the furnace have economic advantages of high carolies, simple installations, easy handling, etc. On the other hand, since heat is carried by the combustion gas into the furnace to heat articles, a large quantity of combustion gas has to be introduced into the furnace. This causes the same quantity of gas to be discharged from the furnace. Unless the replacement is efficient, the fuel is wasted. Then especially significant problem exists in the manner by which inlet and outlet ports for an article to be carried by a conveyor line are provided in the furnace.
In view of this, there has been proposed the circulation method of recovering the heated gas discharged from the furnace and forcing it back into the furnace. This method requires installations, such as a blower for the circulation and ducts, or a heat-exchanger. This disadvantages of such circulation method are increased numbers of components and complicated structure. In the case where such circulation method is used in the heating furnace for backing paint coating, the thinner evaporated from the paints becomes denser while circulated, and unpreferably makes painted surfaces opaque. If the thinner becomes much denser, there will be a hazard of explosion. In order to prevent these situations, air has to be added to make the thinner less denser. Besides, the heat dispersion from the ducts for the circulation is large, and no heat efficiency increase is attained.
There are cases in which the furnace body has to be long in view of a relationship between a speed of the conveyor line and a required heating time. In such cases, the furnace body may be inverted-cup-shaped. In such furnace body, however, the flow of the exhaust gas is so fast that turbulent flows are generated in the furnace body with the result that the temperature distribution becomes nonuniform, and the stable heating is accordingly impossible. Even in such a case, a uniform temperature distribution can be managed to some extent by contriving to arrange the burner. Depending on heating temperatures (fuels), however, it cannot be managed, where more gas stagnates in the central portion of the furnace body, and in the case that the heating furnace is used for baking paint coatings, the bad effect to painted surfaces and hazards of explosions described above are present.
In a heating furnace of the type wherein an article is heated by introducing into the furnace such hot gas as combustion gas produced by a gas burner or gas heated by an electric heater, since a large quantity of hot gas is constantly introduced into the furnace, the same quantity of the gas is exhausted. For this reason the heat efficiency of the furnace can be improved by decreasing as far as possible the heat quantity removed from the furnace by the exhaust gas. According to a preferred method, hot gas is sequentially admitted into an inverted-cup-shaped furnace for discharging from the lower portion of the furnace the gas whose temperature has decreased by natural convection. With this construction, high temperature gas always fills the upper portion of the furnace and cooled gas is discharged so that a high heating efficiency can be realized. Typical examples of the furnaces of this type are disclosed in Japanese Patent Publication Nos. 12513/1983 and 42225/1984 filed by the same applicants as this invention, the latter being shown in FIG. 8 of this application. As shown in FIG. 8, the heating furnace comprises a main body having a top horizontal portion a and downwardly inclined side portions having inlet/exit ports b and c, a conveyor d for conveying article e to be heated through the furnace, and a source of heat g (which may be a gas burner, a steam heater or an electric heater) located on the bottom wall f of the horizontal portion a. The upper edges of the inlet/outlet ports b and c are located at the same or a little higher level than the bottom wall f. With this construction, the gas of the lowest temperature is discharged through the inlet/outlet ports b and c and the interior of the furnace is filled with high temperature gas, thereby efficiently heating the article e.
Although the heating furnace shown in FIG. 8 can efficiently utilize the cross-sectional area of the furnace, it is relatively difficult to install the source of heat and the belt conveyor. In addition, maintenance of the furnace is difficult and expensive.